1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device constructed by placing a display layer between electrodes, such as a liquid crystal display device, for realizing halftone display.
2. Description of the Background Art
Liquid crystal display devices are hitherto used widely as display devices in various computers and word processors. The liquid crystal display device is designed to change the liquid crystal between an active state and an inactive state by applying or not applying a voltage across electrodes between which a liquid crystal layer is placed. Whether the liquid crystal is in an active state or in an inactive state depends on whether the applied voltage is above or below the threshold voltage determined by the physical properties of the liquid crystal being used. Thus, the liquid crystal display device is, basically, designed for binary display.
On the other hand, in order to enhance the display quality, there is an increasing demand for technology relating to displaying each picture element in halftone in the liquid crystal display device. In the vicinity of the threshold voltage, there exists an intermediate state between the active state and the inactive state of the liquid crystal, that is, the halftone display state. Such halftone state is a transient state between the active state and inactive state of the liquid crystal and is an extremely unstable state which is difficult to control and use in the halftone display.
Meanwhile, in a liquid crystal display device of, for example, twisted nematic (TN) type for basically presenting binary display as stated above, it is known that halftone display can be spuriously realized by making use of characteristics of the human visual sense.
Conventionally, for realizing such halftone display, the following principles have been known.
1. While a user is recognizing the image in the liquid crystal display device, it is difficult for the user to independently recognize the individual elements of lightness, hue and saturation of each pigment element composing the image. In accordance with this aspect, one picture element of the liquid display device is composed of tiny plural picture elements.
2. According to a visual characteristic of a human, it is impossible to clearly distinguish binary state changes of each picture element of shorter than, for example, 1/10 second. Only the change in the mean brightness in a specific time is recognizable. By making use of this aspect, each picture element is changed over between the active state and the inactive state at very small time intervals of less than 1/10 second.
On the basis of these principles, the apparent half-tone display may be realized. As practical techniques utilizing these two principles, the following methods are known. Some of these methods are, however, defined by the present applicant.